edit: Also, setting MPD as 90 is not really going to work as well because MPD is a multi-threaded application. So the main process is running at 90 but the other two threads may not be (I don't think it's doing). The main process isn't doing much, so having it run at at a higher priority doesn't seem to do anything for me.
The best way to tune MPD is to use the extra options way. Refer to the walkthroughs and manuals on how to get this setup correctly.